it looks like there went something wrong merging some blocks.
Just do some additional splits in the blocking the bad region to find the mistake. Perhaps you have to delete the bad block and built it new.

To prevent this sort of problem in future, you need to think more about the topology of the geometry rather than trying to block it as it lays and then merging away verts until it fits.

In this case, your blocking is a simple T-Pipe with a collapsed tipů You should only need to merge a single node pair right at the very end. Everything else should just be to split out that simple shape, delete the blocks that are not part of the topology and then fit (associate) the blocking to the geometry.